DESCRIPTION: A new type of resonator for EPR spectroscopy was developed with support of Innovative Technology Grant (RR09882). The crossed-loop resonator (CLR) consists of two, virtually-independent, lumped-element resonators that have a common sample volume where the loops of the two resonators meet orthogonally. The first resonator excites the spins, and the second resonator detects only the signal caused by the spin system. The phase noise of the source is reflected back to the source and is efficiently isolated (70 dB) from the EPR signal. The CLR eliminates the need for the microwave circulator used in conventional spectrometers to direct the EPR signal to the detector. The CLR essentially eliminates the source phase noise in the detected signal, increases spectrometer sensitivity, and allows dispersion spectra to be measured in the same high signal to noise (S/N) as absorption spectra. Because the baseline of the signal is stable with the CLR, magnetic field modulation can be replaced with superheterodyne detection, which will not cause the signal distortion (passage effects) that occur with field modulation. For pulse experiments the CLR greatly decreases the deadtime of the instrument and allows measurement of a portion of the signal previously inaccessible. Eliminating the circulator allows putting a low-noise amplifier near to, and cooled along with, the sample, greatly increasing S/N. It is now proposed to further develop the CLR in three major ways that are important for EPR measurements of biological samples, particularly at cryogenic temperatures: 1) Show that superheterodyne detection will provide a less distorted signal and higher signal-to-noise (S/N) than field modulation, 2) Show that eliminating the circulator and putting a low-noise amplifier near the sample will provide optimum S/N, and 3) Demonstrate that the design can be extended to X-band and incorporated into commercially available spectrometers and cryostats.